Remote signaling system for train control

ABSTRACT

A remote signaling system is described for the monitoring and control of one or more moving trains from a remote wayside location by means of a vehicle carried antenna and signal transmitter coupled with a track-side cable and designed to introduce a movement variable periodic phase shift into the coupled signals from the train back to the wayside located receiver to enable the desired train position monitoring function. The train carried antenna is magnetically coupled to this track-side cable arrangement such that undesired signal changes due to train vehicle rock and sway motion are minimized, and signal transmission difficulties introduced by the physical position variation of the track-side cable wires relative to the track path are minimized. The track-side cable comprises two closely spaced signal transmission lines each having a crossover at predetermined length intervals, with the physical crossover of each transmission line being equally spaced and substantially half-way between the adjacent physical crossovers of the other transmission line. An alternate embodiment employs a signal signal transmission line or cable having crossovers at predetermined intervals at the track-side. This track-side transmission line is driven by an oscillator. Two train carried antennae, spaced a predetermined longitudinal distance apart, receive a movement variable periodic phase shift signal to enable the desired position monitoring function.

United States Patent 1191 Thorne-Booth June 19, 1973 [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Dec. 24, 1969 [21] Appl. No.: 887,825

Related US. Application Data [63] Continuation-in-part of Ser. No. 637,684, May 11,

1967, abandoned.

52 US. Cl 246/122, 179/82, 246/8 51 int. Cl B611 25/00 [58] Field of Search 246/187 B, 167 R,

246/187 R, 182 R, 63 c, 8, 122 R; 180/98; 179/82 [56] References Cited UNITED STATES PATENTS 392,775 11/1888 Barrett 174/33 FOREIGN PATENTS OR APPLICATIONS 1,455,381 l/l969 Germany 246/122 R 1,286,533 l/l969 Germany 246/63 C OTHER PUBLICATIONS Everitt, Communication Engineering (1937) pp. 334-335.

Primary ExaminerRobert G. Sheridan Assistant ExaminerGeorge H. Libman AttorneyF. I-I. Henson, R. G. Brodahl and M. F. Oglo [57] I ABSTRACT A remote signaling system is described for the monitoring and control of one or more moving trains from a remote wayside location by means of a vehicle carried antenna and signal transmitter coupled with a trackside cable and designed to introduce a movement variable periodic phase shift into the coupled signals from the train back to the wayside located receiver to enable the desired train position monitoring function. The train carried antenna is magnetically coupled to this track-side cable arrangement such that undesired signal changes due to train vehicle rock and sway motion are minimized, and signal transmission difficulties introduced by the physical position variation of the trackside cable wires relative to the track path are minimized. The track-side cable comprises two closely spaced signal transmission lines each having a crossover at predetermined length intervals, with the physical crossover of each transmission line being equally spaced and substantially half-way between the adjacent physical crossovers of the other transmission line.

An alternate embodiment employs a signal signal transmission line or cable having crossovers at predetermined intervals at the track-side. This track-side transmission line is driven by an oscillator, Two train carried antennae, spaced a predetermined longitudinal distance apart, receive a movement variable periodic phase shift signal to enable the desired position monitoring function.

4 Claims, 7 Drawing Figures TRAlN RECElVE R P SCHMITI r86 UTILIZATION TR| CGER I TEQUI PMENT 9O P107 PHXSE SENSITIVE DETECTOR our I 111 GF FILTER PHiASE PHASE MA 7- a. v.

PREAMPLIFIER PREAMPLIFIER W &TRANSIENT &TRANSIENT 1 1 CLIPPER CLIPPER 1 1 I .829. E 2 swiv- 81 i 11- 7 9 81 so {1; 9 |a X x X y X 1 1 ,L/2 u" m w o 05C] LIATOR Patented June 19, 1973 3,740,549

2 Sheets-Sheet l TRANSMITTER 2 2o RECE'VER Iijiif FIG 2 TRAIN wAYsIoE ,2 3O PRIOR ART TRANSMITTER 'f Q I4 34 3e 42 i 44 [3: 3E PHASE m 40 SDENSITIVE Elll FILTER G. 3. I6 36 54 3i PHASE l6 SENSITIVE 56 g DETEcToR ,42

Id fi TRAIN so RECEIVER FIG 4.

7 WAYSIDE 5o TRANSMITTER /7O l4 X X A K .I' LIG TRAIN 76 TRAIN TRANSMITTER TRANSMITTER WITNESSES. INvENToR 5 7% fizz/W George M. Thorne-Booth REMOTE SIGNALING SYSTEM FOR TRAIN CONTROL CROSS REFERENCE TO RELATED APPLICATIONS The present invention is a continuation-in-part of a copending patent application entitled Remote Signaling System for Train Control (changed by amendment to Relative Displacement Information System) by George M. Thorne-Booth, Ser. No. 637,684, filed May 11, 1967, now abandoned.

The present invention is related to the invention covered by U.S. Pat. No. 3,551,889 entitled Remote Signaling of Control Systems by C. S. Miller Ser. No. 637,683, filed May 11, 1967, and the invention covered by U.S. Pat. No. 3,562,712 entitled Remote Transmission of Control Signals by G. M. Thorne- Booth and C. S. Miller, Ser. No. 637,723, filed May 1 1,

1967, both of which are assigned to the same assignee as is the present patent application.

BACKGROUND OF THE INVENTION It is necessary for the automated control operation of a passenger carrying train vehicle, since human life and property are involved and a substantially fail-safe operation of same is desired, to monitor and control very precisely the position and movement of the train vehicle by means of at least one wayside control station. For this purpose information signals are required from the train to the wayside located control equipment and from the wayside located control equipment to the train. It is necessary to instruct continuously each train on an individual basis in regard to its movement and its position. Equipment reliability, signal coupling effects, cost of apparatus and like considerations are important.

It has been known in the prior art to employ magnetically coupled amplitude variable signal transmission techniques between a train carried antenna and the signal conductors leading to the wayside control location, but these techniques have suffered in particular from undesired coupling variations due to the rock and sway of the train antenna relative to its track path and further from difficulty to avoid position variations of the involved signal conductors relative to the track path and the position of the train carried antenna. It was also difficult to construct the prior art antennas to enable effective and economic noise shielding practices.

SUMMARY OF THE PRESENT INVENTION The present invention employs an improved signal conductor arrangement including two pairs of signal transmission conductors or cables, each pair having a crossover located at predetermined spaced intervals to result in a phase change of the signal induced into that pair of cables by a magnetically coupled train carried antenna which moves longitudinally along the length of the pair of cables. In reference to the resulting pair of signals induced in the respective cables, they are alternately in phase and out of phase relative to each other, as sensed by a signal receiver placed at the terminal end of the two pairs of cables. The two pairs of cables are short-circuited at the end remote from this receiver, with the receiver including a crystal filter for each pair of cables tuned to the frequency of the signal induced into its pair of cables. The signals are multiplied together in a phase sensitive detector to provide train movement and train position information for the wayside control equipment. A filter bandwidth in the order of 8 Hertz is adequate for one contemplated embodiment, having crossovers if the transmission cables spaced in relation to the expected top speed of the train such that induced signal phase reversals occur at frequencies up to 8 per second, to accept the fundamental frequency signal from each pair of cables or signal channel and to allow for all crystal filter tolerances involved. The two pairs of cables can be placed side-byside, or one on top of the other, and in close enough proximity that adequate signal coupling from the train carried antenna to each pair of cables is provided.

An alternate embodiment of the invention employs a single pair of conductors or cables having crossovers located at predetermined spaced intervals along its length and driven by an oscillator so that a phase change of magnetic field occurs along the length of the pair of conductors. The resulting far magnetic field about the cable is alternately of one and the other of reverse phases. A pair of train carried antennae are carried by the train in a predetermined longitudinally spaced relationship. A distance of separation equal to one-half the interval of crossover is used in one contemplated embodiment, although any distance except a distance equal to the crossover interval or a multiple thereof may be used. The'signal received by the antenna will alternately be in phase and out of phase, and are multiplied together in a phase sensitive detector to provide train movement and train position information.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows the train to wayside signal transmission cable arrangement of the present invention;

FIG. 2 shows one train to wayside signal transmission cable arrangement of the prior art.

FIG. 3 shows one contemplated signal receiver te rmination for the train to wayside signal transmission cable arrangement of FIG. 1;

FIG. 4 shows one contemplated wayside to train signal transmission circuit arrangement;

FIG. 5 shows one suitable manner to effect signal coupling in relation to the relative positions of different portions of the same train.

FIG. 6 shows an alternate embodiment of invention including diagrammatic illustration of certain alternate position situations and their relationship to a waveform; and

FIG. 7 illustrates a preferred form of transmission line having crossovers.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present remote signaling system is operative with a fixed signaling block communication system for train control in which the immediate position of each train is defined by its presence in one such signaling block, with the length of a block being as short as a couple hundred feet or as long as 3,000 feet. As shown in FIG. 1 a loop antenna 10 is adapted to be carried by a train vehicle and driven by a high powered oscillator or train transmitter 12, such that the loop antenna 10 passes over and is magnetically coupled to each of the first pair of signal conductors 14 and a second pair of signal conductors 16. Each pair of signal conductors is provided with physical cross-overs 18 as shown in FIG. 1 at regular length intervals, such that approximately 35 feet of spacing is provided between each cross-over of each cable, with the cross-overs of the pair of cables 14 occurring at substantially half-way between the locations of the adjacent cross-overs of the pair of cables 16. Thusly longitudinal movement of the train carried antenna along the length of the transmission cables causes variation of signal coupling resulting in a relative phase reversal each time that the antenna 10 passes over one of the cross-overs 18. These relative signal phase reversals are sensed by a wayside receiver 20 to monitor the train position and the train movement in terms of velocity and acceleration through signals supplied by the train carried antenna 10.

In one contemplated embodiment of the remote signaling system shown in FIG. 1 the individual conductors of the pair of cables 14 are in the order of 2 inches apart and are to be laid between the track rails upon which the train moves. The loop antenna 10 is carried at the head end of the vehicle and driven from a crystal controlled oscillator, such as the train transmitter 12, and is positioned in proximity such that it is magnetically coupled to both of the transmission cables 14 and 16. The transmitter 12 is operative at a frequency in the order of 10 KHZ and induces into each transmission cable 14 and 16 signals having the same frequency. As the train vehicle moves along the length of the transmission cables, the resulting induced signals in the two transmission cables are alternately in phase and out of phase as the loop antenna 10 moves over the cross-over locations 18 of the respective cables 14 and 16. The transmission cables 14 and 16 are shown side by side in FIG. 1 however it should be understood that they can be positioned one on top of the other. It is contemplated that in the practice of the present invention the spacing between the antenna 10 relative to the position of the transmission cables 14 and 16 may vary in the order of 3 to inches and a lateral or sideways misplacement of the transmission cables relative to the intended location of same in the order of 6 inches is not unlikely. It is contemplated for one embodiment of this invention that the train transmitter 12 will be effective to satisfactorily couple an induced signal into the transmission cables 14 and 16, such that the position and movement of the train can be reliably monitored, with the power of the train transmitter 12 being in the order of 30 microwatts.

In FIG. 2 there is shown one train to wayside signal transmission cable arrangement of the prior art, wherein a Greek square wire transmission cable 24 is provided at a location adjacent the track and preferably between the two rails of the track such that a train carried antenna 26 is magnetically coupled with the Greek square wire in regard to signals transmitted from a train oscillator 28. With the train carried antenna 26 in the position shown in FIG. 2, a maximum coupling is effected to induce a maximum amplitude signal into the transmission cable 24 which maximum amplitude signal is received by a wayside receiver 30. As the train carried oscillator moves along the transmission cable and assumes the position 26 shown by dotted lines, the induced signal attains a minimum amplitude value in regard to the signal induced into the transmission cable and received by the wayside receiver 30. Thusly the movement and position of the train carried antenna 26 can be monitored by the wayside receiver in terms of available pulses which can be counted, the number of such pulses indicating the amount of movement along the length of the transmission cable and the frequency of said received pulses indicating the movement velocity and acceleration of the train carried antenna 26. The arrangement of FIG. 2 is more susceptible in regard to the movement of the train carried antenna in a direction perpendicular to the length of the transmission cable 24 due to rock and sway of the train as it moves along the track and further requires a substantially higher power level of the train oscillator 28, in the order of 50 watts peak, for a single train vehicle to be reliably monitored by the wayside receiver 30.

In FIG. 3 there is shown one contemplated train to wayside signal receiver termination for the signal trans mission cable arrangement shown in FIG. 1 and including transmission cables 14 and 16. Coupling transformers 34 and 36 are provided to assure desired DC signal isolation, with the induced signal from the transmission cable 14 passing through a crystal filter 38 and the induced signal from the transmission signal 16 passing through a crystal filter 40 which are tuned to the fundamental frequency, in the order of IOKHz, of the respective channels and have a bandwidth in the order of 8 Hertz to pass the phase reversal rate. If desired, signal limiter circuits can be provided to remove noise components of the received signals. A phase sensitive detector 42 provides the output pulse signal 44, in the order of 8 Hertz, illustrated in FIG. 3, which pulse signals can be counted to monitor the position of the train carrying the antenna 10 shown in FIG. 1 and further can be followed in regard to frequency variation for monitoring the movement in terms of velocity and acceleration of the train carried antenna 10 shown in FIG. 1.

In FIG. 4 there is shown one contemplated wayside to train signal transmission circuit arrangement of the present invention, whereby a wayside transmitter 50 induces substantially equal communication signals into the transmission cable 14 and the transmission cable 16, which cables are short circuited at their end remote from the phase sensitive detector 42 and are connected through a physically separated and remote conductor 52 to complete the signal transmission path relative to the wayside transmitter 50. The coupling transformers 34 and 36 are provided with center tapped primary windings and it is to the center taps 54 and 56 of these respective windings that the signal provided by the wayside transmitter 50 is connected. This results in an equal balance of signal energy induced into each conductor of the transmission cables 14 and 16 and in effect four parallel conductors for these signals are provided thereby relative to a train carried antenna 58 which is operative with a train receiver 60 for receiving the command signals from the wayside transmitter 50. These command signals can be in the form of frequency modulated pulse coded speed command signals for the train and, for example, are utilized to instruct the train in regard to whatever movement or operation of the train is desired after the actual position and movement of the train is known through the above described operation of the apparatus shown in FIGS. 1 and 3. These command signals are described in greater detail in the above referenced copending applications.

FIG. 5 shows one suitable manner to effect signal coupling in relation to sensing the relative positions of different portions of the same train. A first antenna is provided at the head end-of the train and a second antenna 72 is provided at the rear end of the train, with the antenna 70 being operative with a train transmitter 74 operating at frequency A and the antenna 72 being operative with a different train transmitter 76 having a frequency B to thereby provide a means for sensing the length of the train. The induced signals from the antenna 70 as sensed by the terminating wayside located receiver are in accordance with the position of the antenna 70 along the length of the transmission cables 14 and 16 as are the induced signals from the antenna 72 effective to make the wayside located receiver aware of the physical location of the antenna 72. Therefore any position difference between the antennas 70 and 72 sensed by the wayside receiver is in accordance with the length of the train. The direction of motion of the train can be determined by the change of the position of the respective antennas 7t and 72. If for some reason the head end of the train became separated from the tail end of the train, the resulting positional difference change between the sensed positions of the antennas 70 and 72 would indicate the relative positions of the head end and trailing end of the train, and relative to each antenna its direction of motion may be deduced by the wayside receiver as are the velocity and acceleration obtainable by differentiation of the received pulse signals.

It should be noted that the train to wayside signal transmission cable arrangement shown in FIG. 1 has the substantial advantage that it is more certain in its operation and requires less power to provide a more reliable monitoring and control of the train associated with the antenna and the train transmitter 12.

Reference is now made to FIG. 6, which shows an alternate embodiment of a position monitor system in accordance with the present invention. An oscillator or transmitter 78 generates a continuous wave signal which is driven into a periodically transposed cable 79 in fixed position on the railway track wayside. The physical configuration of this cable is like the configuration of those in FIG. 1. However, here the cable is used as a transmitting antenna, but the principles of transportation are the same, providing a phase of electromagnetic field which alternately reverses itself with each consecutive transposed section of the cable. This is symbolically depicted in FIG. 6 for four of the transposition sections of cable 79 by phase symbols below those sections. The cable is used to monitor position relative to a stop position adjacent a passenger platform 79a. A pair of spaced receiving antennas 80 and 81 are carried by the railway vehicle and are positioned where they pass above the transposed cable. The receiving antennas are positioned sufficiently close above the cable then when an individual antenna is above a transposition section of cable, it will pick up signals from that section in preference to signals from other sections of the cable. It is to be understood, however, that the receiving antennas have enough distance separating them from the cable to couple therewith in accordance with the well known principles of far field electromagnetic behavior.

In the illustrated situation the interval of transposition is L, and the fixed longitudinal spacing of the two receiving antennas is L/2. As the receiving antennas move along the cable, they will respectively receive signals which are alternately in-phase and out-of-phase with each other, depending on the position over the transposed cable.

The signals picked up by the receiving antennas are fed to two separate channels. The respective channels include preamplifier and transient clipper stages 82a, 82b, crystal filters 83a, 83b, and amplifier-limiter stages 84a, 84b. The two channels are then brought together in a phase sensitive detector 85. Detector 85 is of a type which detects the relative in-phase and out-of-phase relationship of the signals in the two channels, such as the type in which these signals are multiplied together. The output of the phase sensitive detector is a signal indicating either the in-phase or the out-of-phase relationship of the two channels. This signal is fed to a Schmitt trigger circuit 86, which squares up the signal to make it suitable as a digital input to utilization equipment.

This mode of operation is diagrammatically illustrated in FIG. 6 by the phantom line alternate positions 81 and 80", 81" of the pair of antennas, and the waveform 88. The waveform 88 represents the instantaneous Schmitt trigger output conditions corresponding to the position of the centerline between the pair of receiving antennas 80, 81 relative to the cable 78. This center has been shown by the dashed vertical lines 89, 89" positioned midway between the receiving antennas for alternate positions 80, 81' and 80", 81", respectively. The positions 80', 81 of the antennas are such that they will pick up in-phase signals from the cables, and at this instant the output of the Schmitt trigger circuit will be at its HIGH logic state, as shown by the waveform. As the vehicle moves along the cable to the right, the antennas will move to positions 80", 81", Where they then receive signals from sections such that the received signals are out-of-phase, and the Schmitt trigger output is at its LOW state, as shown. As the railway vehicle moves along the track, the output of the Schmitt trigger circuit will be double the frequency at which the vehicle passes transposition sections. This signal is then passed to the equipment which utilizes the position of vehicle formation. An example of equipment which utilizes this signal is a mass transit vehicle positioned stop system, such as disclosed in US. Pat. No. 3,519,805, Ser. No. 686,512, filed Nov. 29, 1967 by George M. Thorne-Booth entitled Vehicle Stopping Control Apparatus. That equipment counts the crossover of transposed cables as the train passes over a cable adjacent the station. The count is used for the purpose of controlling speed to achieve a desired deceleration to stop at a particular position alongside the platform. In one contemplated use of the present invention with that system, a transposition interval of L=l 2 inches is used.

The relationship between transposition interval and distance between the receiving antennas has been illustrated in the drawing as a ratio 2:1, and this produces a square wave, as shown. However, it can be shown that a pulsed output corresponding to the count of passed crossovers will be generated for a variety of distances of separation of the antennas, including separation distances which are less than L, and distances which are greater than L. The desired pulsed output will not be produced if the separation distance is L itself, or an exact multiple of L.

The transposed cables illustrated in FIGS. 1, 5 and 6 are configured with the individual wires bent diagonally at each crossover location. This has been done to better illustrate the concept of crossover. In practical embodiments it has been found that cable configuration in which the conductors are bent substantially transversely to the axis of the cable, as illustrated by cable 90, FIG. 7, produces a sharper waveform.

While preferred embodiments of the invention have been described, it should be understood that the various modifications and changes in arrangement of parts may be within the scope and spirit of the present invention.

I claim as my invention:

1. In a vehicle control system wherein a vehicle travels along a predetermined path, apparatus for producing a signal indicative of vehicle position, comprising:

signal transmission means including an alternating current signal source operatively connected to a first inductive transmitter loop; signal receiving means including second and third inductive receiver loops forming separate receiving channels, with said second and third receiver loops being substantially coextensive, elongated loops aligned with said path, said first transmitter loop being operated to couple with both the second and third receiver loops, with at least one of the second and third receiver loops being formed by first and second longitudinal conductor portions having a predetermined arrangement pattern comprising successive lateral transpositions of said first and second conductor portions at corresponding longitudinal intervals of said at least one of the second and third receiver loops, to inductively receive a signal from the first transmitter loop in alternately one and the other of opposite phase relationships as the first transmitter loop passes therealong;

one of said signal transmission means and said signal receiving means being in fixed position relative to said vehicle path. and the other being movable in position with travel of the vehicle;

said first, second, and third inductive loops having predetermined cooperative physical arrangements in a direction aligned with said path such that the signals induced into the second and third loops are alternately in an in-phase and in an out-of-phase relationship to one another under travel of the vehicle along said path; and

means for comparing the phase of signal induced into the second and third inductive loop to derive an output signal representative of vehicle position.

2. In a vehicle control system wherein a vehicle travels along a predetermined path, apparatus for producing a signal indicative of vehicle position, comprising:

signal transmission means including an alternating current signal source operatively connected to a first inductive transmitter loop;

signal receiving means including second and third inductive receiver loops forming separate receiving channels, and being operative to couple with said first transmitter loop, with said second and third receiver loops being coextensive elongated loops aligned with said path, said second and third receiver loops each being formed of a pair of longitudinal conductor portions successively laterally transposed at predetermined periodic intervals, with the periodic intervals of the second and third receiver loops being longitudinally displaced relative to one another to respectively receive signals which are alternately mutually in-phase and mutually out-of-phase under travel of the vehicle;

one of said signal transmission means and said signal receiving means being in fixed position relative to said vehicle path and the other being movable in position with travel of the vehicle;

said first, second, and third inductive loops having predetermined cooperative physical arrangements in a direction aligned with said path such that the signals induced into the'second and third loop are alternately in an in-phase and in an out-of-phase relationship relative to one another under travel of the vehicle along said path; and

means for comparing the phase of signal induced into the second and third inductive loop to derive an output signal representative of vehicle position.

3. In a vehicle control system, wherein a vehicle travels along a vehicle travel path, apparatus for producing a signal indicative of said vehicles position along said vehicle travel path comprising:

two two-wire transmission lines situated along said vehicle travel path, with each of said two two-wire transmission lines each having regularly recurrent line transpositions and being adjacent each other with a position shift relative to said recurrent line transpositions;

signal transmission means located on said vehicle, in-

cluding an inductive transmitter loop aligned relative to said two two-wire transmission lines for inducing signals therein that are alternately in an inphase and in an out-of-phase relationship relative to one another as said vehicle travels along said vehicle travel path; and

means for comparing the phase of the signals induced into said two two-wire transmission lines, respectively, for deriving said signal indicative of said vehicles position along said vehicle travel path.

4. The combination claimed in claim 3, wherein the transposition interval of each two-wire transmission line is equal. 

1. In a vehicle control system wherein a vehicle travels along a predetermined path, apparatus for producing a signal indicative of vehicle position, comprising: signal transmission means including an alternating current signal source operatively connected to a first inductive transmitter loop; signal receiving means including second and third inductive receiver loops forming separate receivinG channels, with said second and third receiver loops being substantially coextensive, elongated loops aligned with said path, said first transmitter loop being operated to couple with both the second and third receiver loops, with at least one of the second and third receiver loops being formed by first and second longitudinal conductor portions having a predetermined arrangement pattern comprising successive lateral transpositions of said first and second conductor portions at corresponding longitudinal intervals of said at least one of the second and third receiver loops, to inductively receive a signal from the first transmitter loop in alternately one and the other of opposite phase relationships as the first transmitter loop passes therealong; one of said signal transmission means and said signal receiving means being in fixed position relative to said vehicle path and the other being movable in position with travel of the vehicle; said first, second, and third inductive loops having predetermined cooperative physical arrangements in a direction aligned with said path such that the signals induced into the second and third loops are alternately in an in-phase and in an out-of-phase relationship to one another under travel of the vehicle along said path; and means for comparing the phase of signal induced into the second and third inductive loop to derive an output signal representative of vehicle position.
 2. In a vehicle control system wherein a vehicle travels along a predetermined path, apparatus for producing a signal indicative of vehicle position, comprising: signal transmission means including an alternating current signal source operatively connected to a first inductive transmitter loop; signal receiving means including second and third inductive receiver loops forming separate receiving channels, and being operative to couple with said first transmitter loop, with said second and third receiver loops being coextensive elongated loops aligned with said path, said second and third receiver loops each being formed of a pair of longitudinal conductor portions successively laterally transposed at predetermined periodic intervals, with the periodic intervals of the second and third receiver loops being longitudinally displaced relative to one another to respectively receive signals which are alternately mutually in-phase and mutually out-of-phase under travel of the vehicle; one of said signal transmission means and said signal receiving means being in fixed position relative to said vehicle path and the other being movable in position with travel of the vehicle; said first, second, and third inductive loops having predetermined cooperative physical arrangements in a direction aligned with said path such that the signals induced into the second and third loop are alternately in an in-phase and in an out-of-phase relationship relative to one another under travel of the vehicle along said path; and means for comparing the phase of signal induced into the second and third inductive loop to derive an output signal representative of vehicle position.
 3. In a vehicle control system, wherein a vehicle travels along a vehicle travel path, apparatus for producing a signal indicative of said vehicle''s position along said vehicle travel path comprising: two two-wire transmission lines situated along said vehicle travel path, with each of said two two-wire transmission lines each having regularly recurrent line transpositions and being adjacent each other with a position shift relative to said recurrent line transpositions; signal transmission means located on said vehicle, including an inductive transmitter loop aligned relative to said two two-wire transmission lines for inducing signals therein that are alternately in an in-phase and in an out-of-phase relationship relative to one another as said vehicle travels along said vehicle travel path; and means for comparing the phase of the signals induced into said twO two-wire transmission lines, respectively, for deriving said signal indicative of said vehicle''s position along said vehicle travel path.
 4. The combination claimed in claim 3, wherein the transposition interval of each two-wire transmission line is equal. 